Deformable mirrors are going to find applications in optical compensators for correcting the wave aberration of light actively, displays, and optical switches for use in optical communication. Recently, there are a number of R & D reports that a bunch of micromirrors be formed by a micro-machining technique to cut down the cost. Those micromirrors are often driven with an electrostatic force. A type of deformable mirror, driving the micromirrors with an electrostatic force, has a planar and simple structure, and therefore, is highly compatible with a semiconductor device manufacturing process. In addition, as the size of each member such as a mirror is reduced, the electrostatic force produces greater driving force per unit area.
However, in utilizing an electrostatic force, basically only the attraction works unlike the case of using a piezoelectric element. Thus, the driving direction is asymmetric. Also, the driving force is not generated due to the expansion or shrinkage of the structure of the driving portion itself. Accordingly, a pair of electrodes, which is arranged so as to face each other with a gap provided between them, and a structure for supporting this pair of electrodes are needed.
Deformable mirrors are roughly classifiable into “continuous mirrors”, of which one continuous mirror surface is deformed with a lot of actuators, and “segmented mirrors”, in which a number of mutually separated mirrors are moved independently of each other.
In an exemplary continuous mirror, a potential difference is created between a thin-film mirror and a lot of electrodes, which are arranged so as to be spaced apart from each other, thereby producing an electrostatic force, attracting the deformable mirror toward the electrodes and deforming it (see, for example, G. Vdovin, P. M. Sarro and S. Middlehoek, “Technology and Applications of Micromachined Adaptive Mirrors”, J. Micromech. Microeng. 9 (1999), R8 to R20).
Examples of segmented mirrors include a mirror in which micromirrors are attracted with an electrostatic force parallel to the substrate (see, for example, W. D. Cowan, M. K. Lee, B. M. Welsh et al., “Surface Micromachined Segmented Mirrors for Adaptive Optics”, IEEE Journal of Selected Topics in Quantum Electronics Vol. 5, No. 1, pp. 90–101 (1999)) and another mirror that causes a tilting operation, not the parallel shifting operation (e.g., Japanese Laid-Open Publication No. 2001-174724). A segmented mirror that uses a stacked piezoelectric element performs the parallel shifting operation and tilting operation at the same time (see, for example, R. K. Tyson, “Adaptive Optics Engineering Handbook”, Marcel Dekker Inc. (2000) (Chapter 5, p. 155, FIG. 2)).
However, these deformable mirrors have the following drawbacks.
The continuous mirror can generate a continuous and smooth wavefront, and therefore, can reduce the wavefront fit error even with a small number of actuators. Nevertheless, it is difficult for the continuous mirror to secure a control range and reduce the drive voltage at the same time. That is to say, to secure a sufficient control range, the gap distance between the mirror and the electrodes needs to be increased. However, the magnitude of the electrostatic force is inversely proportional to the square of the gap distance. Thus, no electrostatic force can be obtained unless a high voltage is applied. In addition, the continuous mirror has a lower response speed than the segmented mirror, and the residual stress of the mirror film seriously affects the planarity of the mirror.
On the other hand, the segmented mirror is more advantageous than the continuous mirror in terms of the response speed and planarity. Also, the respective mirrors are independent of each other. Accordingly, even if the control range exceeds a wavelength λ, an arbitrary control range is still achieved by performing a shifting process of converting the excessive wavelength back to its associated principal value falling within the range of 0 to λ. As a result, the reduced voltage and broad control range are achieved at the same time.
However, the conventional segmented mirror attracts and shifts the respective mirrors parallel and can produce only a stepped wavefront. Accordingly, to reduce the wavefront fit error, the number of mirrors segmented should be much greater than that of the continuous mirror, thus increasing the number of actuators, too. As a result, the control structure gets too much complicated or the area of the gap between the mirrors increases with respect to the area of each mirror. Consequently, the reflection efficiency decreases and the effects of unwanted diffracted light increase.
The segmented mirror that uses a piezoelectric element can produce a smooth wavefront by performing the parallel shifting and tilting operations simultaneously, thus minimizing the wavefront fit error with just a small number of mirrors segmented. But this segmented mirror has no compatibility with a semiconductor device manufacturing process and is hard to mass-produce at a low cost.
It should be noted that it is difficult to realize the configuration including the piezoelectric element with an electrostatic deformable mirror for the following two reasons:
Firstly, it is difficult for the electrostatic deformable mirror, producing only the attraction, to control the parallel shifting operation and the tilting operation independently.
Secondly, unlike the piezoelectric element that generates a driving force by its own structural expansion or shrinkage, the electrostatic deformable mirror needs a pair of electrodes, which is arranged with a gap provided between them, and a structure for supporting the electrodes as separate members. Thus, it is difficult to provide the pair of electrodes, of which the area is broad enough to obtain a big electrostatic force, and the supporting structure, which enables both the parallel shifting operation and tilting operation alike, within the limited areas of segmented mirrors of such a deformable mirror.
In order to overcome the problems described above, an object of the present invention is to provide a deformable mirror, which can minimize the wavefront fit error with just a small number of mirrors segmented, and an optical controller including such a deformable mirror.